1. Field of the Invention
The present invention relates to vacuum thermal insulators, and particularly to vacuum thermal insulation vacuum thermal insulators having good thermal insulation property which is maintained for a long time.
2. Description of Related Art
Vacuum thermal insulators that have a structural body which is sealed by a container or a wrapping material composed of a gas barrier material and which has a substantially vacuum space inside has been known. Also, a vacuum thermal insulator which has core materials filled in the structural body in order to improve its thermal insulation property and maintain its shape also has been known. Such vacuum thermal insulators can attain high thermal insulation property by maintaining the inside of the structural body at a high vacuum degree to minimize heat transmission caused by gas. In order to maintain such high thermal insulation property for a long time, it is important to form the structural body of a material which has excellent gas barrier property.
Resin, particularly thermoplastic resin, is a preferable material for the structural body in view of its good moldability. However, even PVDC (polvinylidene chloride) and EVOH (a saponified ethylene/vinyl acetate copolymer), which are representative resins of high gas barrier property, are not satisfactory to be used for vacuum thermal insulators and they can not maintain the high thermal insulation property for a long time.
For example, Japanese unexamined patent publications Nos. Shou 63-279083 and Shou 63-233284 disclose that a laminate in which aluminium foil is laminated on a thermoplastic resin film can produce a structural body having good gas barrier property. A vacuum thermal insulator composed of such a laminate can maintain a high vacuum degree for a long time. However, metals such as aluminium have relatively high heat conductivity comparing to resin and air. For example, although heat conductivity of polypropylene resin and air, respectively, are about 0.23 W/mxe2x80xa2K and about 0.02 W/mxe2x80xa2K, that of aluminium is about 200 W/mxe2x80xa2K. Accordingly, a vacuum thermal insulator composed of a metal laminate causes xe2x80x9cheat bridgingxe2x80x9d which is a phenomenon that heat is transmitted in the metal layer and the thermal insulation property drastically deteriorates. The attempt to suppress the heat bridging by thinning the metal layer has resulted in many pinholes opening in the metal layer and has caused deterioration in the gas barrier property and maintainability of the property for a long time.
The present inventors have studied to develop thermal insulators having good thermal insulation property which is maintained for a long time, and have accomplished the present invention.
The present invention provides a vacuum thermal insulator comprising a shell which has a vacuum space inside and has at least one layer (hereinafter referred to as a xe2x80x9cGB layerxe2x80x9d) which contains a resin and satisfies the formula (1):
Wxe2x80xa2xcexP less than 1xc3x9710xe2x88x926xe2x80x83xe2x80x83(1) 
wherein W is the thickness (m) of the GB layer, xcex is the heat conductivity (W/mxe2x80xa2K) of the GB layer and P is the oxygen permeability (cc/m2xe2x80xa2dayxe2x80xa2atm) of the GB layer at 23xc2x0 C. in a humidity of 50% RH.